Studies on the effect of imaging parameters on dynamic mode decomposition of time-resolved schlieren flow images

Abstract

Dynamic modal analysis enables new insights into the spatio-temporal dynamics of complex flow scenarios. Time resolved schlieren imaging provides significant information in compressible flow scenarios on flow structures and their evolution. We conduct a systematic study using synthetic images and experimental schlieren images on the effect of image acquisition parameters on the modal analysis by dynamic mode decomposition (DMD). We consider the effect of two important capture parameters – the capture rate (fs) and the exposure time (texp). Analysis is carried out on two sets of synthetic images, SI-I, an unsteady wavy interface created using a linear combination of sinusoids, and SI-II – hypothetical shock oscillations. Finally, a flapping supersonic jet is observed using high-speed schlieren with a nano-pulsed laser light source with three different imaging parameters. We find that among the two parameters the effect of exposure time on modal analysis and its interpretation is more pronounced than capture rate. An exposure time of 5% of maximum exposure produces 8% reduction in mode amplitude, and in case of long exposure the dynamic significance of modes undergoes complete change. If the flow images are instantaneous, then the spatial mode shapes of dominant modes remain the same irrespective of the capture rate. Aliasing has to be considered in sub-Nyquist capture rates, however, the actual frequencies can be suitably resolved.